Aluminum is a common material for semiconductor device (semiconductor die) bond pad metallization since its introduction to the semiconductor world, including for discrete transistors and integrated circuits (IC). However, aluminum and high aluminum alloys quickly form a refractory ceramic oxide on the surface that requires special treatment to remove before achieving successful soldering thereto. This step is applied to most of the methods in making interconnections from the aluminum bond pad on the semiconductor die built on a semiconductor substrate, usually on single-crystal silicon.
The semiconductor die is packaged in a hermetically sealed case or a non-hermetic plastic capsule, with leads extending from bond pads on the chip to the leadframe. In an electroless nickel immersion gold (ENIG) Under Bump Metallization (UBM) process, the surface treatments starts with surface cleaning to remove any contaminant present on the aluminum bond pad surface, followed by an activation process to activate the bond pads for better nucleation for subsequent processing through the micro-etching of aluminum oxide.
A pre-plating process known as zincation is commonly used. During the zincation process, aluminum oxide is removed and is replaced with a thin layer of zinc metal. The zinc protects the aluminum from re-oxidation until it is ready to be plated. Once this process is performed, the aluminum coated zinc can then be electrolessly plated with nickel followed by immersion gold.